This invention concerns a method for cleaning the SiO2 particles, by heating a fill of the particles in a reactor with a vertically oriented center axis and thereby exposing it to a treatment gas which is conducted at a given flow velocity from the bottom to the top through the reactor and the fill.
Furthermore, the invention concerns a device for the implementation of the method according to the invention, comprising a reactor with a vertically oriented center axis for accepting a fill of SiO2 particles, with a gas inlet for feeding a treatment gas into an area of the reactor essentially below the fill, and with a gas outlet for discharging the treatment gas from an area of the reactor above the fill.
Moreover, the invention concerns cleaned SiO2grain from naturally occurring raw material.
From SiO2 particles, quartz glass products are molten for the chemical and optical industry and for the manufacture of optical fibers and semiconductors. There are high requirements on the purity of the quartz glass products. Especially alkaline metals, alkaline earth metals, heavy metals, iron, carbon and free or combined water may have a detrimental effect on the desired properties of the quartz glass products. Correspondingly high are thus the purity requirements for the raw materials of quartz glass products. Quartz glass raw materials within the meaning of this invention are amorphous or crystalline particles, for example SiO2 particles of naturally occurring quartz, or contaminated synthetically produced grains, granulates, or recycling material.
A method for the continued cleaning of quartz powder by means of thermochlorination has been described in EP-A1 737 653. It has been suggested therein to continuously feed the quartz powder to be cleaned, having a mean grain size ranging between 106 xcexcm and 250 xcexcm, into an electrically heated quartz glass rotary furnace in which it will pass in succession through a preheating chamber, a reaction chamber and a gas desorption chamber. In the preheating chamber, the quartz powder is heated to approx. 800xc2x0 C. and is subsequently treated in the reaction chamber at a temperature of about 1,300xc2x0 C. with a gas mixture of chlorine and hydrochloric gas. The alkaline and alkaline earth contaminations of the quartz powder here react with the chloric gas mixture, forming gaseous metal chlorides. The treatment gas and the gaseous reaction products will subsequently be exhausted.
The known cleaning method leads to a significant reduction of alkaline and alkaline earth contaminations in the quartz powder. The purity of the quartz powder can be yet improved even further by repeated passage of the cleaning process. However, in many quartz powder applicationsxe2x80x94such as for example as a starting material for quartz glass components in the use of semiconductor manufacture or for opticsxe2x80x94the purity of the starting materials will be subject to extremely high requirements which cannot be achieved with the known method or only under great expenditure of time, material and cost.
With the known method, the cleaning effect depends on the reaction period of the quartz powder with the chloric gas mixture and on the reaction temperature. At higher temperatures, chlorine reacts faster with the metallic contaminations so that a better cleaning effect would have to be expected with increasing temperatures. However, at high temperatures, agglomerates will form due to the softening of the grain which will interfere with further access of the treatment gas to the surface of the individual grains. The cleaning effect by the treatment gas which primarily acts on the grain surface will thus be reduced. Furthermore, the cleaning effect depends on the dwell period of the quartz powder in the reaction chamber. Coarse grain powder usually passes the reaction chamber faster than fine grain powder. Thus, different purities may result which may even be different within one charge, depending on the temperature, the grain fraction or throughput. This complicates the reproducibility of the known cleaning method.
In the cleaning method according to DD-PS 144 868, fluid quartz sand Is continuously charged from above into a vertically oriented reactor. The quartz sand fill passes the reactor continuously from top to bottom. The quartz sand fill therein passes a heating up zone, a thermochlorination zone and a cooling down zone. To avoid oxygen from penetrating into the chlorination zone and thus to prevent the reformation of the chlorides formed during chlorination into metal oxides, an inert gas or nitrogen gas curtain is produced at the entrance area and at the exit area of the chlorination zone.
Wet chemical treatments are also customary for removal of contaminations on the surface of naturally occurring quartz sand. In such a method as is described for example in U.S. Pat. No. 4,983,370, the quartz sand is pretreatedxe2x80x94prior to the cleaning process by thermochlorinationxe2x80x94first by means of a two-stage flotation process, a magnetic separator and a subsequent caustic treatment in hydrofluoric acid.
A cleaning method and a device for the implementation of the method in accordance with the aforementioned species are known from EP-A 1 709 340. Therein is suggested to continuously feed SiO2 powder, manufactured by means of flame hydrolysis, to a vertically oriented reactor to remove chlorine, and to treat the powder fill in a countercurrent with a gas mixture of water vapor and air which is conducted through the reactor from the bottom to the top. In the area of the fill, the gas mixture has a linear gas velocity ranging between 1 and 10 cm/s and a temperature ranging between 250xc2x0 C. and 600xc2x0 C. The gas flow will flow through the fill forming a so-called xe2x80x9cfluidized bedxe2x80x9d, and the fill will be slightly raised.
It has been shown that the degree of purity of the SiO2 particles such as it is required for use in semiconductor and optical fiber manufacture cannot be achieved by means of the known methods. Especially the contaminations with the chemical elements Li, Na, K, Mg, Cu, Fe, Ni, Cr, Mn, V, Ba, Pb, C, B and Zr cannot be sufficiently removed by means of the known methods.
This invention is accordingly based on the task of providing an improved method for cleaning the SiO2 particles and of also providing a suitable simple device therefor; as well as of specifying SiO2 grain from naturally occurring raw material and cleaned by using the method according to the invention, such grain being particularly suitable especially for the manufacture of semi-finished or finished quartz glass products for semiconductor and optical fibers production.
In view of the method, this task is solved in accordance with the invention such that a chlorous treatment gas is being used thatxe2x80x94in the area of the fillxe2x80x94is adjusted to a treatment temperature of at least 1,000xc2x0 C. and to a flow velocity of at least 10 cm/s.
A chlorous treatment gas is used. Many of the contaminations contained in SiO2 particles react with the treatment gas at temperatures of above 1,000xc2x0 C. to form gaseous metal chlorides or other volatile compounds which can be exhausted from the reactor via the exhaust gas. Aside from a chlorous component, the treatment gas may contain additionally other componentsxe2x80x94such as fluorine, iodine or bromine, inert gases or hydrogen for examplexe2x80x94which are specially suitable for the removal of specific contaminations or for adjustment of specific properties of the SiO2 or for the heat transfer between treatment gas and particles. For economic reasons, free chlorine is undesirable in the treatment gas so that the chlorous component will contain chlorine in a combined but reactive form.
The treatment gas will be passed through the fill at a flow velocity of at least 10 cm/s. This will ensure that gaseous compounds of the contaminations will be removed as fast as possible from the particles and discharged from the reactor. Moreover, due to the fast gas exchange, the SiO2 particles are continuously and fast provided with unspent treatment gas so that the rate of chemical reaction between treatment gas and contaminations is as high as possible. Decisive will here be the flow velocity in the area of the fill. It should here be noted that, with the fill, the free flow cross-section versus the empty reactor will be reduced and the flow velocity accordingly increased. Here, and in the following, xe2x80x9cfillxe2x80x9d is to mean a bulk deposit of still to be cleaned SiO2 particles in the reactor.
In the area of the fill, a treatment temperature of at least 1,000xc2x0 C. is adjusted for the treatment gas. Contaminations can generally be removed from the particles the easier and faster, the higher the treatment temperature. Since contaminations react with the treatment gas on the free surface of the particles, it is required that the contaminations come to the surface. This is done essentially by diffusion. The contaminations show specific diffusion speeds in the SiO2 particles; however, a higher treatment temperature will principally also achieve a higher diffusion speed. Moreover, due to the higher temperature, the speed of reaction between treatment gas and contaminations is increased. However, at very high treatment temperatures, there is the risk that the particles will agglomerate in the fill so that not only the fluidity of the fill will be reduced but also the cleaning effect of the treatment gases will be reduced as well due to fewer surfaces being free. With regard to this, there is an upper limit for the treatment temperature of about 1,400xc2x0 C.
The treatment gas will be adjusted to the treatment temperature. SiO2 particles will be heated up by the treatment gas to the treatment temperature or kept at the treatment temperature. Accordingly, the temperature of the treatment gas is at least as high as that of the particles. Thus will be prevented that gaseous components condense out of the treatment gas and thus will also be prevented a concurrent adsorption or absorption of contaminations on the particles or, respectively, into the particles.
The method is suitable for a batch-wise as well as a continuous cleaning of the particles. Taking into account the specific reaction temperatures for contaminations with Li, Na, K, Mg, Cu, Fe, Ni, Cr, Mn, V, Ba, Pb, C, B and Zr, the purities achievable with the method according to the invention are always in a sub-ppb range.
It has proven particularly advantageous to adjust the temperature of the treatment gas to at least 1,200xc2x0 C. in the area of the fill. The higher the treatment temperature is adjusted in the area of the fill, the easier and faster contaminations can be removed for the reasons above explained in detail. The treatment temperature from which a significant reaction can be observed between a contaminating element and the treatment gas (reaction temperature) is specific to the element. Thus, sodium contaminations for example can already be significantly reduced at temperatures of approx. 1,000xc2x0 C. with the chlorous treatment gas, whereas higher temperatures of at least 1,050xc2x0 C. are more favorable for the removal of lithium contaminations.
Advantageously, the treatment gas is introduced into the fill such that it will lift it by producing a fluidized particle layer (fluidized bed). The treatment gas will largely flow laminarly through the fluidized particle layer. This will achieve a homogeneous gas distribution in the fill so that the particles are homogeneously charged with the treatment gas. Blind spots in the area of the SiO2 particle layer will be avoided as far as possible so that the contaminations are able to react completely and uniformly. This will achieve a cleaning effect by the treatment gas which is done as completely and uniformly as possible. Moreover, due to the fact that the individual particles of the fill are kept moving in the fluidized bed, the risk of sintering of the particles will be reduced so that the treatment temperature can be set higher.
Maintenance of the most laminar flow possible within the fill will be made easier if the treatment gas is already introduced into the fluidized particle layer in the form of the most laminar gas flow possible. This procedure wherein the laminar flow is already produced below the fill of the particles to be cleaned has proved advantageous especially for the removal of lithium contaminations.
A procedure is preferred wherein the treatment gas is heated, prior to its introduction into the fill, to the treatment temperature or to a temperature above the treatment temperature. Here, the treatment gas will expand to a multiple of its volume at normal temperature. The volume increase is concurrent with a corresponding increase of the flow velocity of the treatment gas. This will facilitate the formation of a fluidized particle layer and a largely laminar flow of the treatment gas through the fill. Moreover, due to the preheating of the treatment gas, the above mentioned condensation effects will be avoided. Due to the contact with the somewhat colder SiO2 particles, the treatment temperature in the treatment gas will come about in the area of the fill.
It proved to be particularly advantageous to use pure inorganic hydrochloric gas as the treatment gasxe2x80x94apart from the inert gases. Such treatment gas contains in particular the lowest possible components with an oxidizing effectxe2x80x94such as oxygenxe2x80x94so that the formation of chlorine in the waste gas of the treatment gas can be largely prevented. This will make a chlorine degassing of the waste gas unnecessary so that its disposal or recycling will be simplified. For example gas scrubbers can be done without, such as they are used with the known cleaning processes for chlorine degassing. Advantageously, such hydrochloric gas contains a stoichiometric excess of elementary hydrogen. Excess hydrogen will react with OH groups in or on the SiO2 particles to form water which is carried as water vapor with the gas flow so that the OH content of the particles can be reduced.
It proved to be successful to introduce the treatment gas into the fill by means of a gas shower having numerous nozzle openings below the fill which are distributed laterally to the center axis. Underneath the fill of the particles to be cleaned, the gas shower has nozzle openings which are essentially symmetrically distributed over the cross-section of the fillxe2x80x94viewed in direction of the flowxe2x80x94and from which the treatment gas will flow. Due to the gas flow applied in an area at the start of the fill, the fill is laminarly passed through in a more or less straight line, and a uniform and homogeneous treatment of the particles over the entire fill will be ensured.
Advantageously, the particles are heated to a temperature in the area of the treatment temperature with the exclusion of air and oxygen. With this procedure, any oxygen or air existing in the fill or in the reactor will be replaced by another gas prior to the heating of the particles, for example by an inert gas or by a treatment gas which, apart from contaminations, is free of oxygen and nitrogen. This will prevent that stable combinations of contaminations in the form of nitrides or oxides will be formed at higher temperatures which can no longer be removed subsequently by the treatment gas. Due to the air and oxygen exclusion, a redox reaction with HCl-containing treatment gas is suppressed which can result in the formation of chlorine gas. As already explained further above, the processing or degassing of the treatment gas would then become considerably more expensive.
In a preferred method, the treatment gas is simultaneously used for air sifting of the fill. For many applications, the removal of the fines content of a grain is desirable since the fines may result in fine bubbles of the quartz glass. By adjusting the flow velocity of the treatment gas, this fines content can be defined and removed from the fill and discharged from the reactor. At the same time, a defined grain fraction can thus be obtained.
Advantageously, a first cleaning stage will be provided for the removal of metallic contaminations or their compounds, especially sodium, manganese, potassium and iron contaminations, and a second cleaning stage for the removal of carbon and carbon compounds. During the second cleaning stage, an oxygen-containing gas is charged to the treatment gas; whereas in the first cleaning stage, an oxygen containing gas is avoided as far as possible. Carbon could cause gas inclusions during the melting of particles so that, during the second cleaning stage, carbon is made to react with oxygen to CO or CO2 and will be removed from the fill.
In a particularly economic process, the treatment gas is run in circulation. Here, the waste gas leaving the reactor is regenerated and again charged to the reactor as a treatment gas. For balancing any gas consumption, regeneration may comprise an admixture of fresh, unused treatment gas.
One procedure proved especially favorable where the flow velocity of the treatment gas is adjusted to at least 30 cm/s. Due to the high flow velocity, the gas exchange will be Increased. The advantages explained in detail above, with regard to the discharge of contaminations and high reaction speeds between the treatment gas and the contaminations, will be further promoted by maintaining a fluidized bed of the fill (fluidized particle layer).
With regard to the device, the above specified task is solved in accordance with the invention by the gas inlet comprising a gas shower, havingxe2x80x94below the fillxe2x80x94a multitude of nozzle openings laterally distributed toward the center axis, for introducing the treatment gas in the fill.
Due to the fact that a gas shower is arranged below the fill, a laminar flow can be produced below the fill, and the treatment gas can be introduced into the fill in the form of a gas flow which is as laminar as possible. Thus, maintenance of the most laminar flow possible within the fill will be facilitated.
The gas shower comprises a multitude of nozzle openings distributed laterally to the center axis. The nozzle openings are essentially distributed symmetrically about the center axis and uniformly over the cross section of the fillxe2x80x94seen in the direction of flow. Due to the gas flow designed in such a laminar fashion at the beginning of the fill, the gas will flow laminarly and more or less in a straight line through the fill, and a uniform and homogeneous treatment of the particles over the complete fill will be ensured.
Advantageously, the gas inlet comprises a gas heating device which is arranged before the gas showerxe2x80x94seen in the direction of flow of the treatment gas. Thus, the treatment gas can be heatedxe2x80x94prior to introducing it into the fillxe2x80x94to a temperature of above the treatment temperature. The effect and the special advantages of this procedure are explained above in more detail on the basis of the method according to the invention.
Especially simple in design is a gas heating device which is formed as a heated tubular coil. By means of the tubular coil, the adjustment of the treatment gas to the treatment temperature can be easily realized by adjusting their lengths to the requirements. The length of the tubular coil is commonly in the range of a couple of meters. It can be arranged in a furnace in which the reactor is located as well, and it consists of a high-temperature resistant material, preferably quartz glass.
The gas shower also consists of a temperature resistant material, for example of quartz glass, silicon carbide or a precious metal, such as platinum or a platinum alloy. In the simplest case, the gas shower is designed in the form of a tube provided with nozzle openings. The tube may have a multitude of forms, for example, the form of a spiral. The gas shower may also be designed as a perforated plate or a frit.
Production of a laminar gas flow will be facilitated if the nozzle openings of the gas shower are symmetrically distributed about the center axis of the reactor. For example, the nozzle openings can be uniformly arranged in annular form around the center axis, with adjacent nozzle openings advantageously having the same distance to each other.
A reactor proved to be especially advantageous which is closable on all sides. Thus, the introduction of oxygen or nitrogen and the resulting risk of the formation of chlorine and thermochemically stabile compounds of contaminations, such as nitrides or oxides, will be prevented.
With regard to the SiO2 grain, the above specified task is solved in accordance with the invention by the SiO2 grain of naturally occurring quartz having an iron content of less than 20 ppb by weight, preferably less than 5 ppb by weight; a manganese content of less than 30 ppb by weight, preferably less than 5 ppb by weight; and a lithium content of less than 50 ppb. by weight, preferably less than 5 ppb by weight, as well as a content of chromium, copper and nickel of each less than 20 ppb by weight, preferably less than 1 ppb.
The SiO2 grain according to the invention is manufactured from natural quartz grain or pretreated quartz grain such as for example commercially sold precleaned grain or it is manufactured from granulate, with such quartz grain being cleaned by means of the method according to the invention. The natural SiO2 grain thus manufactured excels by purities which are otherwise only achievable with synthetically manufactured SiO2. It is thus especially suited as a material to be used for the manufacture of high-purity quartz glass. For example for the manufacture of quartz glass crucibles, bars, rods, plates which are used as components or semi-finished products for the semiconductor industry, for optics and optical communications systems.